1. Field of Invention
The present invention relates to phosphors, a method for treating the surface thereof and a process for producing a phosphor screen. Particularly, it relates to phosphors for cathode ray tubes having improved coating properties for the preparation of a phosphor screen.
2. Description of Prior Arts
A typical process for preparing a phosphor screen for a color picture tube comprises dispersing a phosphor in a photosensitive resin solution such as polyvinyl alcohol activated by ammonium dichromate, to obtain a phosphor slurry, coating this phosphor slurry on a glass panel, and irradiating ultraviolet rays in a prescribed pattern through a shadow mask to cure and insolubilize the resin. Then, with a developer such as warm water, the portions not irradiated with ultraviolet rays are dissolved and removed. This operation is repeated three times with respect to three colors (blue, green and red) to form a phosphor screen in the form of stripes or dots.
In the process for forming such a phosphor screen, the phosphor is required to satisfy the following conditions:
(1) The phosphor screen must be dense and have little pin holes or the like.
(2) One light emitting phosphor component must not intermix with other light emitting elements (stripes or dots), i.e. cross color contamination must not occur.
(3) Formation of the light emitting elements must be excellent, i.e. the edges of the stripes must be straight, the edges of dots must be circular, and the edges must be sharp.
(4) The phosphor photosensitive solution must have a high level of light exposure sensitivity, and the photocuring speed by ultraviolet rays is high so as to provide good working efficiency.
(5) During the process of exposure and development, the portions cured by irradiation with ultraviolet rays must have strong bond strength, and the phosphor screen at such portions must not fall off from the glass panel by high pressure aqueous development treatment.
A phosphor having zinc hydroxide deposited thereon is known as a phosphor having conditions (2) and (4) improved among the above conditions. (U.S. Pat. No. 4,287,257)
In recent years, a finer image is required for a color picture tube in order to improve the image quality. For this purpose, it is necessary to form fine picture elements (stripes or dots). In order to consistently form fine picture elements on a glass panel, it is important that the bond strength of the phosphor to the glass panel is strong and no cross color contamination will take place. As a method for increasing the bond strength of the phosphor, there may, for example, be mentioned a method wherein a phosphor suspension coated in a process step prior to the development is forcibly dried to form a strong phosphor layer. However, there is a problem that the stronger the phosphor layer prior to the development, the more likely fogging will result on other colors.
The above mentioned phosphor having zinc hydroxide coated thereon has a limit in the level of forming a highly fine image. Further, even if the amount of zinc hydroxide coated is increased, no adequate improvement is attainable in the prevention of fogging. On the contrary, such an increase tends to increase the coagulation of the zinc hydroxide colloid, whereby the dispersibility of the phosphor in the suspension tends to be poor, thus leading to a decrease in the bond strength or a deterioration such as breakage of the edges or formation of pin holes.